The present invention relates to electrochemical cells and, more particularly, to nonaqueous cells utilizing a polymer electrolyte.
Solid-state cells having a lithium anode, a finely-divided metal oxide cathode and a polymer electrolyte possess many advantages over other types of electrochemical cells. For example, such cells provide a long useful service life, possess a relatively high power density, a relatively high specific energy--on the order of 200 to 300 Wh/kg--and a relatively high specific power--on the order of 600 W/kg. Such cells are also desirable in that their polymer-based electrolyte component can be readily and inexpensively formed with greatly reduced thickness, and the entire cell can be shaped in a number of desirable configurations thereby making the cell useful in a wide variety of applications.
An example of such a cell is disclosed in the publication Advanced Battery Development, published in 1984. That cell comprises a lithium foil anode, a cathode of finely divided vanadium oxide with carbon black additions mixed with electrolyte material, termed a "composite electrode," and an electrolyte comprising an ion-conducting polymer consisting of poly(ethylene oxide) complexed with the lithium salt LiCF.sub.3 SO.sub.3 deposited in a thin layer on a nickel foil.
A laminate, comprising elongate strips of anode material, polymer electrolyte, cathode material and current conductor layers is formed into a cell by folding the laminate in a "fan-folded" or "concertina" configuration. However, in the fan-folded configuration disclosed, the stacked segments of the laminate overlie each other such that the segments of current conductor contact each other and the segments of anode material contact each other on successive segments of the fan-folded laminate.
In another embodiment, a cell comprises a spiral or "jelly roll" configuration in which a laminate comprising a strip of insulator material, a strip of anode material, a strip of polymer electrolyte, a strip of composite cathode material and a strip of current collector are wound into a spiral shape. Successive windings of the aforementioned laminate are isolated from each other by the strip insulator segment of the laminate.
Another battery cell construction is disclosed in the article "The Magic of MOLI" in the June 1987 issue of QST. That cell includes a molybdenum disulfide cathode, a polymer electrolyte and a lithium anode. A laminate comprising an anode sandwiched between two electrode layers which, in turn, contact two cathode layers is wound in jelly-roll fashion and encased in a steel can to form the cell. Strips of metal foil extend transversely of the anode and cathode layers and are connected to the negative and positive electrodes, respectively, of the resulting battery.
However, the aforementioned cell designs possess some disadvantages. For example, the lithium/vanadium oxide cell designs disclosed show a single cathode laminated to each anode strip, and a single current conductor for each cathode. Consequently, a portion of the anode strip opposite the electrolyte layer is largely unused and adds unnecessarily to the cost, weight, and thickness of the resultant cell. It is not currently practicable to manufacture lithium anode strips which are sufficiently thin to overcome this disadvantage.
The disadvantage with the MOLI cell is that each cathode requires its own current conductor, again adding to the overall weight thickness and cost of the resultant cell. Accordingly, a need exists for a lithium battery which is relatively inexpensive to manufacture, is efficient in operation in that it uses substantially the entire anode strip and is light in construction.